Hard start, runs great

I have a 1992 XJ, 4.0 4wd with 175k. Well-maintained, in pretty good shape actually.

Just recently replaced all fluids, water pump, steering pump, idler pully and a tranny flush. I also replaced the struts and some other stuff unrelated to the engine.

Anyway, I started hearing a screeching noise coming from the engine bay and the initial diagnosis was the AC compressor. We pulled that and installed a bypass pulley and that was when the engine started giving trouble. As it turns out, it was the steering pump pulley grinding on the bracket so we fixed that and reinstalled the compressor, thinking maybe something we did there was causing the engine to not start. It was starting just fine before the compressor was pulled and replaced with the bypass pulley.

Anyway, putting the compressor back on didn't work. I think the engine is having a hard time turning over because it is running rich. It smells like fuel right after you start it and it smokes a lot for a few minutes. Also when we pulled the plugs they were CAKED in black carbon and they were in good shape not too long ago.

I can get the Jeep started with some vigorous pumping of the throttle. I floor it until the engine starts to catch then I start pumping like mad until it can stay running on its own then I let it settle into its idle. If you gas it the engine will hesistate around 1500rpm, completely lose revs for a split second then rev up however high you want it. Just like if you were revving at a red light. vroom vrooooooooom. I drove it for about 20 miles up and down the freeway and it cleared up and drove great and smoothed out. Stopped to eat at a restaurant, got back out and bam the engine didn't wanna start without the rigamarole with the throttle.

So now it sits, a bitch to start and I have no idea why. I can't think of anything that could have caused this from only removing the AC compressor.

I checked the spark, I checked the air and I checked the fuel. All seem good. Once you get it started it runs great, maybe a little low on power but hard to say. It does seem to be using more fuel than normal.

Would resetting the ECU help with this?

1. Ever seen a check engine light? Even if the answer is now, I would scan the computer for any "stored codes" that may provide a clue. Here is how on your 92..
http://www.go.jeep-xj.info/HowtoECUcodes.htm

2. Test your fuel pressure. Must be done with a gauge; no shortcuts. Something like a ruptured fuel pressure regulator can cause a rich mixture. You should have 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator and the pressure should jump to approximately 39 psi. Be sure that the line leading to the FPS has good vacuum.

3. Test your map sensor. You'll need a meter and a manual. Be sure that the line leading to this sensor is pulling good vacuum. Map sensor is involved with fuel/air ratio

4. Test your throttle position sensor. Having to tip in the throttle to start the engine is a pretty common symptom of a failing TPS.

5. Test you coolant temp sensor. Or just replace it if it doesn't show a very wide swing in resistance between a cold engine and a hot one.

6. Be sure ALL tuneup hardware is fresh. That means plugs, wires, cap and rotor. Importance of a strong secondary ignition cannot be overstated

7. Replace the oxygen sensor if it is old or you have no idea how many miles are on it

1) Pull off the vacuum hose on the front of the fuel rails Fuel Pressure Regulator and crank the engine. If you see, or smell fuel coming out the vacuum port, the FPR is bad, and flooding the engine.

2) If that does not work I would suspect fuel injectors are sticking open, or leaking, not completely closing. So time to pull the fuel injectors next, and test them.

IIRC the HO engine had the Renix FPR up until about 96, OBD-II?

I had similar problems recently, and it was a leaking FPR, leaking fuel into the vacuum line.

So I did the key thing to get the code. It gave me a 12 and a 55. I had reset the ECU the night before so I suppose that's what it was seeing. I have driven it quite a bit since then and it is still showing no codes.

I'll check on a gauge to measure the fuel pressure precisely. I just crapped out most of my cash to get the Jeep where it is at now so things are starting to get tight.

To test the TPS I'll have to grab a new multimeter too. blargh...

Since I really don't know the shape of ANY of the parts you mentioned I may just replace them all, depending on the total cost.


I am going to go step out and check the vacuum port on the fuel rail right now to see if I smell fuel. I'll post back in a few with what I find.

Ok, so I went and pulled the vacuum line from where it attaches on the left side of what I am pretty sure is the FPR. I then turned it over and I neither smelled nor saw any fuel coming from the nipple of the FPR. I reattached and then pulled it from where it attaches to the intake manifold. From the actual hose, leading from the FPR, I do smell fuel.

I really don't have much beyond basic hand tools available to me.

I don't know if it matters, but it was really cold this morning, and after sitting all night it just cranked right up on the first try. Every try after that, however, has been a nightmare.

Gonna go fiddle with the TPS and see if it's just a loose connection or something.

Disconnected all the stuff around the throttle body and reconnected it all. Didn't help.

It definitely smells like fuel under the hood and in the exhaust when you try turning it over.

Sounds like a leaking fuel injector, stuck open, leaking fuel after you power it off. A fuel pressure gauge would confirm that!!!! TPS would not cause this problem in my opinion.

I am leaning towards this as well. What would be the fix? Replace the injectors?

When I checked the plugs all 6 were heavily caked with carbon. Does this indiciate a problem with something other than individual injectors? Would be hard to believe all 6 would pop at once. I do have a cracked exhaust manifold and I haven't checked the plugs in a year. Perhaps that is just from almost a year of running extremely rich?

Could it be something else in the system allowing the fuel to siphon from the lines into the cylinders?

In the morning I will check to see if it starts properly from just sitting overnight.

I will try to see about getting a pressure gauge tomorrow.

Ecomike said:

Sounds like a leaking fuel injector, stuck open, leaking fuel after you power it off. A fuel pressure gauge would confirm that!!!! TPS would not cause this problem in my opinion.

Click to expand...

You may have more than one problem. Stuck, leaking injector or two flooding it at shut down causing the hard starting, flooding.

Cracked exhaust screwing with a good O2 sensor, making it run the engine rich on all cylinders, or bad O2 sensor letting the ECU run the engine rich, causing the black spark plugs. If it has been that rich that long, the Cat may have overheated and been damaged by to much raw fuel, and may now be partly blocking the exhaust adding to your grief, but since you have no high rpm loss (?) the Cat must not be blocked yet?

The Exhaust crack can confuse the O2 sensor and cause the rich running, but will not cause the hard start. The carbon fouled plugs may make starting a little harder, but I think raw fuel from one or more bad injectors is the starting problem now.

When you get it fixed, be sure to change the OIL!!!!!

Injectors may just need a good cleaning. I test mine with a 9 volt battery and I just blow through them to check for open, close and sealing.

I do agree that each thing you mentioned is definitely an issue. However, this problem came very suddenly out of nowhere. While the cracked manifold is almost certainly causing the engine to run rich generally, I can't really afford to replace it right now. I'm gonna stick some of that thick black muffler tape around the collector part of the exhaust manifold in the hopes it helps a bit with the leak. It's been running rich like this for two years and for now it is a lot easier to replace $2 plugs every few months than it is to replace the exhaust manifold right now. I'm hoping to rebuild it next year anyway so figured that's when I'd bother with the manifold.

I think it is cylinder 3 or 4 that is not firing up properly and causing the extremely hard starts.

I don't know how to clean or test injectors. How on earth do you use a 12v battery to test the injectors? I'm broke but I will pick up a fuel pressure gauge later in the week when I get more money if I still need one. In the meantime is there anything I can be doing that might help narrow down the precise problem? I'm really hoping this is an easy fix and I don't need to throw money at it especially since I am limited to basic hand tools (I don't own any torx bits, jack stands, jacks etc. I have screwdrivers, some sockets, some wrenches and that's about it). I have to pay a mechanic for any major work.

I just started the Jeep. First start of the day, fired right up. As soon as the ignition made contact it started running. It didn't even have to turn over once before it caught.

Idle is rough, but smoothing out.

Later starts in the day will most likely be very hard.

Would now be a good time to upgrade the injectors? I still have OEM.

Idle once smooth and warm is around 900rpm. When you let off the gas from revving it takes a long time to drop RPMs to idle (about 3 seconds or so to drop from say 1600rpm --> 1000rpm).

I would go borrow (loan) a pressure gauge for free from the parts store (you just need to make a refundable security deposit at Advanced, AZ or O'reileys, or just borrow it at the store and test it running at the store. Cost - 0.00, just a trip to the store. Then pick up a set of fuel injector orings, and some cheap torx bit wrenches, or tips while you are there, or buy some beer and call of one of the many Naxja people that live nearby to come help.

Pull the fuel rail and injectors as an assembly. If you pull the injector electrical plugs, NUMBER them first, with tape!!!!

I test my injectors by using a standard 9volt battery, and a pair of rinky dink radio shack jumper wires. If the injector clicks when you apply voltage, and if you can blow air though the injector with your mouth while it has power, it passes test one. Then with no power, see if you can still blow air through the injector. If not, then the injector is good, otherwise you have the leaky injector!!!

When you reinstall the injectors, use the NEW ORINGs!!!

Ecomike said:

1) Pull off the vacuum hose on the front of the fuel rails Fuel Pressure Regulator and crank the engine. If you see, or smell fuel coming out the vacuum port, the FPR is bad, and flooding the engine.

Click to expand...

This is going to sound retarded but I want to clarify - the vacuum port on the FPR, right?

I didn't think they'd have something like that as a loaner. Good to know, and thanks for the tips. I can easily just check the pressure there.

Changing the injectors sounds easy peasy. I figured it would require some specialized tools to do but if that's all it takes then I'm set.

So if I have a leaky injector there is no fixing it, all I can do is replace it right? Those things are expensive.

Ecomike said:

I would go borrow (loan) a pressure gauge for free from the parts store (you just need to make a refundable security deposit at Advanced, AZ or O'reileys, or just borrow it at the store and test it running at the store. Cost - 0.00, just a trip to the store. Then pick up a set of fuel injector orings, and some cheap torx bit wrenches, or tips while you are there, or buy some beer and call of one of the many Naxja people that live nearby to come help.

Pull the fuel rail and injectors as an assembly. If you pull the injector electrical plugs, NUMBER them first, with tape!!!!

I test my injectors by using a standard 9volt battery, and a pair of rinky dink radio shack jumper wires. If the injector clicks when you apply voltage, and if you can blow air though the injector with your mouth while it has power, it passes test one. Then with no power, see if you can still blow air through the injector. If not, then the injector is good, otherwise you have the leaky injector!!!

When you reinstall the injectors, use the NEW ORINGs!!!

Click to expand...

I search but could not find a procedure for how to test the fuel line pressure. What do I do once I get a hold of the pressure gauge?

Thanks for all the help so far.

It should screw onto the schraeder valve on the fuel line.

Ok, got it. So I don't actually disconnect any fuel lines, it just checks pressure at the valve.

So I take it then that after I connect the gauge, I get the vehicle running and see what the pressure reads? Anything else I need to do?

rvmccarthy said:

Ok, got it. So I don't actually disconnect any fuel lines, it just checks pressure at the valve.

So I take it then that after I connect the gauge, I get the vehicle running and see what the pressure reads? Anything else I need to do?

Click to expand...

No, you shouldn't. Do as suggested above and pull the vacuum hose and observe the readings then, too.

Also, you can prime the pump w/o starting and see how the pressure holds, just for shits and grins since you'll have it all connected.

Maybe I'm missing something but no one has given me any information on how to use the pressure gauge. I don't see anything suggested above about how to use it...

It's all in this thread, read it again.

birchlakeXJ said:

2. Test your fuel pressure. Must be done with a gauge; no shortcuts. Something like a ruptured fuel pressure regulator can cause a rich mixture. You should have 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator and the pressure should jump to approximately 39 psi. Be sure that the line leading to the FPS has good vacuum.

Click to expand...

Not using the fuel gauge but something you should do anyways.

Ecomike said:

1) Pull off the vacuum hose on the front of the fuel rails Fuel Pressure Regulator and crank the engine. If you see, or smell fuel coming out the vacuum port, the FPR is bad, and flooding the engine.

Click to expand...

Then checking your injectors if it goes this far:

Ecomike said:

Pull the fuel rail and injectors as an assembly. If you pull the injector electrical plugs, NUMBER them first, with tape!!!!

I test my injectors by using a standard 9volt battery, and a pair of rinky dink radio shack jumper wires. If the injector clicks when you apply voltage, and if you can blow air though the injector with your mouth while it has power, it passes test one. Then with no power, see if you can still blow air through the injector. If not, then the injector is good, otherwise you have the leaky injector!!!

When you reinstall the injectors, use the NEW ORINGs!!!

Click to expand...